When a white sauce tastes starchy and grainy, the likely cause is:

The correct answer indicates that an uncooked flour is responsible for the starchy and grainy taste in a white sauce. In the preparation of a white sauce, also known as a béchamel, flour is typically cooked in fat (a process known as making a roux) before liquid is added. This cooking step is crucial because it helps to gelatinize the starch in the flour, allowing it to thicken the sauce smoothly without creating a grainy texture.

If the flour remains uncooked, the starches have not been properly hydrated and gelatinized. As a result, they won't dissolve fully when the liquid is added, leading to that undesirable starchy and grainy mouthfeel. Cooking the roux adequately ensures that the sauce has a silky texture, whereas skipping this step results in a sauce that lacks the desired smoothness.

Understanding this process reinforces the importance of following proper techniques in cooking, particularly when creating sauces that rely on the smooth incorporation of starches for thickening and texture.